Seamless porous metal article and method of making

ABSTRACT

A method for making seamless, porous metal articles comprising: 
     (a) rotating a mold containing a stabilized suspension of a metal particulate at a rate and for a time such that the particulate is separated from the suspension and distributed on the interior wall of the mold, thereby forming a structure conforming to the interior wall of the mold, the rate of rotation being sufficiently high that, preferably, at least about 65 Gs up to 100 Gs of centrifugal acceleration is achieved at the interior wall of the structure, 
     (b) drying the formed structure to provide a structure having green or unsintered strength, and 
     (c) sintering the dried structure to remove volatile material and fuse the individual particles of the particulate to each other to form the seamless, porous metal article. 
     Metal articles in accordance with the invention have substantially uniform diameters, thicknesses and pore structurs, have Bubble Point ratios of greater than 1.5 up to 2.5, and find particular use as filters. 0044

This application is a continuation-in-part of U.S. application Ser. No.935,644 filed Nov. 26, 1986, which in turn is a continuation of U.S.application Ser. No. 697,391, filed Feb. 1, 1985, both now abandoned.

TECHNICAL FIELD

This invention relates to seamless porous metal articles. Moreparticularly, this invention is directed to seamless porous metalfilters and a method for making them.

BACKGROUND ART

Metal filters have long been used for a variety of applications. Forexample, porous stainless steel filters prepared from sintered metalparticulate, e.g., stainless steel powder, have found use in a varietyof processes where high pressure drops are acceptable and inapplications where relatively fine filtration capability must becombined with mechanical strength, resistance to high temperaturesand/or resistance to chemical attack. Such applications include thefiltration of fine catalysts used in fluidized bed catalytic processeswhere elevated temperatures are encountered, e.g., fluid cat cracking,and in the manufacture of high fidelity recording tapes. Still anotheruse of such filters is in the filtration of molten resin used in themanufacture of polymeric films and fibers as, for example, polyesterfilm.

One form of commercially available metal filters in cylindrical form istypically prepared from sheet material which is formed into acylindrical shape and then longitudinally welded. Unfortunately, thismethod of manufacture results in a structure sensitive to rapidtemperature change, i.e., uneven heating and cooling can ultimatelyresult in cracking and failure of the structure adjacent the seam weld.Other drawbacks to such welded structures are nonuniform blow backcharacteristics and the inability to make relatively small diameterstructures, e.g., at one-half inch diameter, the welded seam occupies asignificant portion of the overall surface available for filtration,limiting the onstream filter life for a given cycle.

DISCLOSURE OF THE INVENTION

In accordance with the subject invention, seamless porous metalarticles, and a method of forming them, are provided which substantiallyovercome the limitations described above with regard to presentlyavailable metal filters of the type described above. Additionally, thearticles in accordance with this invention have uniform porecharacteristics, hence longer onstream life, making them particularlydesirable in filtration applications.

The method in accordance with this invention provides a means forpreparing seamless porous metal articles or structures, particularlyuseful as filters, in which the porosity of the resulting structure canbe accurately tailored by controlling (1) the makeup of the compositionsused to prepare the structure and (2) readily measurable processvariables.

In accordance with the invention, a method is provided for making aseamless, porous metal article comprising:

(a) rotating a mold containing a stabilized suspension of a particulateat a rate for a time such that the particulate is separated from thesuspension and distributed on the interior wall of the mold, therebyforming a structure conforming to the interior wall of the mold, therate of rotation being sufficiently high that at least about 65 Gs up to100 Gs of centrifugal acceleration is achieved at the interior wall ofthe structure;

(b) drying the formed structure to provide a dried structure havinggreen or unsintered strength; and

(c) sintering the dried structure to remove volatile material and fusethe individual particles of said particulate to each other to form aseamless, hollow, porous structure.

Preferably, the mold is completely filled with the stabilized suspensionprior to initiating rotation.

The metal articles in accordance with this invention are seamless,hollow porous structures of substantially uniform diameter, thicknessand pore structure particularly useful as filters comprising metalparticulate in which the individual particles of the particulate arebonded to each other. The porous metal articles have Bubble Point ratios(as defined below) of greater than 1.5 up to about 2.5.

BEST MODE FOR CARRYING OUT THE INVENTION

The stabilized suspension used to prepare seamless porous metal articlesin accordance with this invention is comprised of a liquid medium, ametal particulate, a stabilizing agent and a binding agent. Preferably,a single constituent serves to both stabilize the dispersion of metalparticulate and, upon drying of the suspension, to bind the individualparticles to each other and to the container, thereby providing therequisite green or unsintered strength, i.e., a stabilizing bindingagent is used.

Typically, the stabilized suspension of the metal particulate in theliquid medium is prepared by the following general procedure.

The stabilizing/binding agent is combined with the liquid medium,preferably water for ease of use and disposal, in an amount such as toprovide the requisite concentration of the stabilizing/binding agent inthe liquid medium. For the preferred stabilizing/binding agent, CARBOPOL941, discussed below, the stabilizing/binding agent preferably comprisesfrom about 0.1 percent to about 0.9 percent of the mixture, i.e., thestabilizing/binding agent and the liquid medium. The preferredstabilizing/binding agent is CARBOPOL 941, available from B. F. GoodrichChemicals Company, which provides the medium with a relatively highviscosity. For example, in a CARBOPOL 941/water mixture, where theCARBOPOL 941 comprises 0.35 percent by weight (based on the weight ofthe water), the viscosity is approximately 750 centipoise at 20 degreesCentigrade. When the CARBOPOL 941 comprises 0.9 percent of the mixture(based on the weight of the water), the viscosity of the mixture isapproximately 1,200 centipoise. Mixtures of CARBOPOL 941 and water arepreferred because the combination provides compositions havingsubstantially consistent viscosities, i.e., mixtures of theseconstituents provide compositions with viscosities which are readilyreproducible.

Based on the diameter of the largest particles of the metal particulateto be suspended in the stabilized suspension, a value for the viscosityof the stabilizing/binding agent-liquid medium mixture that will renderthe suspension sufficiently stable can be determined. The desiredviscosity of the stabilized suspension in accordance with this inventionis such that the suspension is capable of holding the metal particulatein suspension and thereby remaining substantially uniformly dispersedprior to lay down under the impetus of the centrifugal force generatedin the rotating container. Knowing the desired viscosity of thestabilizing/binding agent-liquid medium mixture, the relative amounts ofthese constituents to be used in preparing the mixture can bedetermined. It is therefore desirable to use a stabilizing/binding agentwhich will, when mixed with the liquid medium to be used, produce asuspension having relatively consistent bulk viscosity values withregard to the relative amounts of constituents used and relativelyconstant viscosity values throughout the suspension. In general, lessstabilizing/binding agent is used with finer metal particulates. This isdue to the reduced tendency for finer particles to settle out.

The combination of stabilizing/binding agent and liquid medium(sometimes referred to herein as the carrier) is preferably mixed untiluniform dispersion of the stabilizing/binding agent is obtained. Themetal particulate material is then added and mixed with thestabilizing/binding agent-liquid medium mixture to provide a uniformstabilized dispersion or suspension of the metal particulate in thecarrier. The weight ratio of the metal particulate to the carrier, i.e.,the other components in the stabilized suspension, is typically fromabout 8:1 to about 1:1, preferably from about 4.5:1 to about 3.5:1. Thisratio depends primarily on the desired thickness of the porous articleand the interior volume of the mold or container.

The amount of particulate metal powder required for a given metalarticle can be determined by the following relation:

    amount of particulate metal powder (weight) needed=Va·ρ·K,

where

Va=annular volume of the finished seamless porous metal article, i.e.,the volume occupied by the wall of the structure;

ρ=the apparent density of the metal particulate powder; and

K=the shrinkage factor.

The shrinkage factor, K, is determined empirically by measuring the wallthickness of the formed structure before and after sintering.

The viscosity of the stabilized suspension of dispersed metalparticulate is preferably below the gel consistency so that, for ease ofprocessing, the stabilized suspension can be poured. However, a gelledstabilized suspension and a high rate of rotation may be preferable whenrelatively large particles are used.

For some systems, the suspension of metal particulate in the liquidmedium containing the stabilizing/binding agent is stable after athorough mixing has been completed. By stable or stabilized is meantthat the metal particulate material is in suspension and will not settleout at a rate fast enough to adversely affect the formation of thedesired structure. That is, no settling or elutriation of particulateoccurs prior to the initiation of rotation.

For many applications it is preferred to add an additional component toset up the stabilizing/binding agent. For example, the addition of aneutralizing base, ammonium hydroxide, to CARBOPOL 941 serves toneutralize the stabilized suspension and increase the viscosity to asubstantial degree. Such systems are very thixotropic. i.e., they have avery high apparent viscosity when undisturbed (low shear condition) and,hence, settling of the suspended particulate is retarded. Whenvigorously agitated, they have a low effective viscosity and, hence, arevery effective in dispersing the metal particulate. Since thesesuspensions are very stable, they may be prepared in advance of the timethey are used without settling out of the metal particulate.Alternatively, stabilizing/binding agents may also be used which do notrequire the addition of another component to set up the suspension.CARBOPOL 941, the preferred stabilizing/binding agent, may be used withor without the addition of a neutralizing base. For example, withcorrosive-sensitive metal particulate, neutralized CARBOPOL 941 ispreferred because it is less acidic. In other cases, it may bepreferable to use a viscosity-increasing agent to aid in stabilizing thesuspension.

A variety of viscosity-increasing agents, which serve to stabilize themetal particulate suspension and also act as a binding agent when theliquid medium is removed by drying, may be used. Polyacrylic acid(available from B. F. Goodrich Chemical Company under the trade nameCARBOPOL) is particularly desirable. As previously noted, CARBOPOL 941is particularly preferred. CARBOPOL 941 has a molecular weight of about1,250,000. CARBPOL 934 may also be used. It has a molecular weight ofabout 3,000,000. Other materials which can be used include carboxymethyl cellulose, carboxy ethyl cellulose, polyethylene oxide, sodiumcarboxy methyl cellulose, guar gum, alginates, methyl cellulose, andlocust bean gum. In general, when water is used as the liquid medium,water compatible stabilizing/binding agents which volatilize and/ordecompose substantially completely prior to or during sintering may beused.

The metal particulate can be of any of a variety of metal materialsincluding alloys, various metals such as nickel, chromium, copper,molybdenum, tungsten, zinc, tin, gold, silver, platinum, aluminum,cobalt, iron and magnesium, as well as combinations of metals and metalalloys, including boron-containing alloys. Nickel/chromium alloys arepreferred. Of these, the AISI designated stainless steels which containnickel, chromium and iron are more preferred. Particularly preferred arethe AISI 300 series of stainless steels, commonly referred to as theaustenitic stainless steels. Other stainless steels within the preferredclass are the martensitic stainless steels, maraging steels, 17-7 and17-4 PH stainless steels, ferritic stainless steels, and Carpenter No.20 alloy. Other alloys within the preferred class of nickel/chromium arethe Hastelloys, the Monels and the Inconels, as well as a 50 weightpercent nickel/50 weight percent chromium alloy. Multi-structuredmaterials, such as duplexes of ferritic and austenitic stainless steel,may also be used. The metal particulate used may have various shapes,including dendritic, acicular, fibril, and spherical, and will typicallyhave average particle sizes in the range of from about 8 to about 60micrometers. The size of the metal particulate chosen for a particularapplication is related to the porosity in the finished seamless porousmetal article.

The austenitic stainless steel porous articles in accordance with thisinvention are characterized by having low carbon residues, i.e., lessthan about 0.08 weight percent, more preferably less than about 0.05precent, and typically 0.03 percent or less, e.g., 0.015 percent. Lowproduct carbon content is due to the very low concentration of binderresin which, in turn, is made possible by tailoring the weight ratio ofthe metal particulate to carrier (stabilizing/binding agent and liquidmedium) in the suspension. Typically, the amount of carbon present inthe stabilized suspension by virtue of the stabilizing/binding agent isabout 0.25 percent or less (based on the weight of the metalparticulate). Part of this is lost during heat up in the sinteringoperation, and the residual quantity of carbon actually absorbed intothe metal is reduced by chemical or physical processes which occurduring sintering.

A low carbon content is particularly significant when working withaustenitic stainless steels since austenitic stainless steels withcarbon contents greater than 0.08 weight percent are susceptible toprecipitation of chromium carbides at the grain boundaries which cancause corrosion under many conditions. This susceptibility to corrosionis exacerbated when austenitic stainless steel containing greater than0.08 weight percent carbon has been exposed to a temperature in therange of from about 900 to about 1,500 degrees F. (sensitization range).Typically, the lower the carbon content, the lower the susceptibility ofthe austenitic stainless steel to intergranular corrosion. Austeniticstainless steel having carbon contents in the range of from about 0.03to about 0.08 weight percent are stable when they have not beensubjected to a temperature in the sensitization range. However, whensuch steels are exposed to a temperature in the sensitization range,chromium carbides will precipitate at the grain boundaries and the metalthen becomes susceptible to attack by various corrosive media.Austenitic stainless steels with carbon contents less that 0.03 weightpercent will not precipitate significant amounts of chromium carbides atthe grain boundaries even after they have been subjected to atemperature in the sensitization range, thus exhibiting a highercorrosion resistance than comparable austenitic stainless steels withcarbon contents greater than 0.03 weight percent.

The process by which the carbon is removed during sintering fromaustenitic stainless steel porous articles is not fully understood. Ithas, however, been empirically determined that it is generally noteconomically practical to obtain products with less than about 0.015 to0.08 percent of carbon if the starting mixture contains more than about1 percent carbon. This is thought to be the case because, even if thecarbonaceous binder melts and/or volatilizes, enough carbon is diffusedinto the the metal from the liquid or vapor to undesirably increase itscarbon content to levels well above 0.08 percent. For these reasons, theweight percent of the carbon in the stabilized suspension in thesuspending medium to the weight of the particulate austenitic stainlesssteel should preferably be kept to less than about 0.25 percent of theweight of the metal particulate.

Substantially spherical particles may be used to provide a more tightlycontrolled pore size distribution uniformly distributed within thestructure. Alternatively, metal fibers or metal fiber/metal powdercombinations can be employed in the stabilized suspension.

In carrying out the method of this invention, an elongated, hollowcylindrical container is at least partially filled, preferablycompletely filled, with the stabilized suspension of dispersed metalparticulate. The container or mold may be formed of any material capableof withstanding the sintering temperatures employed in the process.Examples of material that can be used include silicon carbide, siliconnitride, molybdenum and various ceramics. However, the coefficient ofthermal expansion of the metal particulate must be substantially greaterthan that of the container or mold. This is necessary to maintain goodsupport for the compacted particulate structure during the sinteringstep. A container or mold with a lower coefficient of thermal expansionthan the metal particulate does not expand as much as the dried metalparticulate structure as the sintering step is carried out. As a result,the metal particulate tends to press into the walls of the mold, therebymaintaining the shape and compacted nature of the structure untilsintering is complete. Preferably, the coefficient of thermal expansionof the metal particulate is at least one and one-half times as great asthat of the container or mold. For example, for the preferred ceramiccontainers of molds, as discussed below, the thermal coefficent ofexpansion generally is within the range of from about 1.0×10⁻⁶ to about4.0×10⁻⁶ inches per inch per degree Fahrenheit. For the preferredstainless steel metal particulate, the coefficient of thermal expansionis generally in the range of from 6.0×10⁻⁶ to about 9.0×10⁻⁶ inches perinch per degree Fahrenheit.

As noted, ceramic tubes are the preferred structures for use as the moldor container. Tight tolerance, cylindrical ceramic tubes are availablewhich will produce very uniform seamless porous metal cylindricalarticles. In addition, ceramic tubes are not adversely affected by thesintering process and the formed metal article does not adhere to theceramic material as a result of the sintering step. Accordingly, thecontainer can be reused. Several examples of compositions of ceramictubes are as follows:

(a) 99.8 percent Al₂ O₃ (alumina) extruded to full density;

(b) 96.0 percent Al₂ O₃ (alumina) extruded to full density;

(c) 85.0 percent mullite and 15.0 percent glass extruded to fulldensity;

(d) 100 percent mullite extruded to full density;

(e) 80 percent Al₂ O₃ and 20 percent SiO₂ slipcast and isotacticallypressed to 80 percent density.

The composition of the ceramic identified by the letter (e) above is themost preferred for use with the present invention. Ceramic tubes of thismaterial exhibit good dimensional tolerances over a relatively longlength. For example, ceramic tubes of this composition are availablewith standard camber (curvature) tolerances for tubes with diametersfrom 1 to 6 inches of no greater than 0.020 inch variation per linearfoot and no greater variations in wall thickness than 0.005 inch, i.e.,the concentricity of the inside diameter to the outside diameter is nogreater than 0.005 inch. As the wall thickness of the ceramic tubesincreases, the diameter tolerance becomes even tighter. The porousarticles formed using ceramic tubes with these tight tolerances havecomparable tight tolerances.

Prior to adding the stabilized suspension of dispersed metal particulateto the container, one end of the container is preferably sealed with,for example, a rubber stopper or other suitable means, e.g., inlaboratory tests, adhesive tape has been used. The amount of stabilizedsuspension of dispersed metal particulate to be added is preferably theamount necessary to substantially completely fill the container. Afilled container is preferred because it provides more uniformdistribution of the metal particulate resulting in a product with a moreuniform pore structure. Additionally, a completely filled container aidsin start up because the center of gravity is more nearly coincident withthe longitudinal axis of the cylinder. After adding the stabilizedsuspension to the container, it is then sealed and mounted on astructure capable of rotating the container about its longitudinal axis,preferably with the container in a substantially horizontal position.For example, a machine lathe, such as a Clausing lathe, or a spindle maybe used. The container or mold is rotated at a high enough rate toprovide a centrifugal acceleration at the interior wall of the formedstructure equal to or greater than about 65, more preferably from about70 to 80 or greater, and may be as high as up to 100 Gs to produceporous articles with Bubble Point ratios of greater than 1.5 up to about2.5. (Copending U.S. application Ser. No. 935,644 filed Nov. 26, 1986discloses the use of forces of 100 Gs or greater to obtain porous metalarticles with Bubble Point ratios of 1.5 or less.) The rate of rotationrequired varies inversely with the diameter of article being formed. Forexample, to generate a centrifugal force of 70 Gs in a two inch diameterporous article, the rate of rotation must be about 1,575 rpm. Similarly,for a one inch diameter article, the rate of rotation must be about2,225 rpm, and for a one-half inch diameter article, the rate ofrotation must be about 3,150 rpm.

It may be desirable to construct a porous metal structure having agraded pore structure, e.g., a transition of pore sizes with thestructure having larger particles, and therefore larger pores, near theexterior wall, and smaller particles, and therefore smaller pores, nearthe interior wall of the structure. One of the ways the method inaccordance with the present invention can be used to provide such astructure is to introduce metal particulate of a broad particle sizedistribution into the stabilized suspension, and then rotate thecontainer initially at a lower rate of revolution, thereby laying down ahigher percentage of larger particles than of smaller particles over theinterior wall of the mold, followed by an increase in the rate ofrotation as the structure is formed, so that the smaller size or finerparticles are distributed over the previously distributed largerparticles so that the final or interior portion of the structure laiddown comprises more smaller particles than it does larger particles anda graded pore structure with an outside-in configuration is formed. Thefinal rate of rotation must be sufficiently high to provide at leastabout 65 Gs of centrifugal acceleration to obtain the desired level ofcompaction which provides the articles in accordance with this inventionwith the desired uniform pore characteristis, i.e., a Bubble Point ratioof greater than 1.5 up to about 2.5. When operating in this manner, alower concentration of metal particulate in the suspension is preferred.

A method for producing a layered structure, contemplated by the presentinvention, comprises laying down a first stabilized suspension on theinterior of the container at a specified rate of rotation, removing thesupernatant liquid, drying the material thus laid down and introducinganother stabilized suspension containing metal particulate having adifferent particle size distribution than the first stabilizedsuspension, i.e., finer (or coarser) particles and repeating theprocedure. A structure having any desired number of layers, with layersof varying pore size, can be produced in this manner. Further, thegradation of pore sizes from larger to smaller can be on either aninside-out or outside-in basis. Indeed, it is also within the scope ofthis invention to provide layers of different pore sizes in alternatingfashion, e.g., a fine pored layer on the exterior of the structure, anintermediate layer of larger pore size, and an inner layer of fine poredmaterial. As discussed above, the final rate of rotation with eachstabilized suspension must be sufficiently high to provide at leastabout 65 Gs of centrifugal acceleration for each particular layer priorto its being dried. A preferred structure prepared by this techniquecomprises an external layer comprised of relatively finer sized metalparticulate, e.g., -325 austenitic stainless steel, and an inner layerof relatively coarser metal particulate, e.g., -200, +325. A structureof this type with a nominal four inch outer diameter and in which theouter layer is about 0.015 inch thick and the inner layer is about 0.040inch thick has properties which make it particularly useful forfiltering gases.

As used herein, this type of nomenclature, e.g., "-200, +325" mesh,refers to the characteristics of the particulate material. In thisspecific instance, -200 means it passes through a 200 mesh U.S. standardsieve while +325 means it does not pass through a 325 mesh U.S. standardsieve. Similarly, the nomenclature "-325" powder size refers to a powderin which all the particles will pass through a 325 mesh U.S. standardsieve.

The container is generally rotated at the desired rpm for from about 3to about 5 minutes, following which it is stopped. Longer times may beused but have not been found to be necessary. Preferably, the containeris allowed to slow down without being stopped abruptly, more preferablyit is allowed to spin until its momentum runs out. The container is thenremoved from the rotating structure, supernatant fluid is removed, andthe formed structure is dried in the container, preferably while in ahorizontal position, to provide the structure with "green" or unsinteredstrength. Drying is preferably conducted in a convection oven at fromabout 100 to about 210 degrees Fahrenheit for about 30 to about 45minutes or longer.

The container is then placed in a furnace, such as a vacuum furnace orreducing atmosphere furnace, most preferably a vacuum furnace, to removevolatile material and to fuse the individual particles of the metalparticulate to each other. The sintering is best done with the structurein a vertical position to avoid distortion due to high creep rates ofthe metal particulate at elevated temperatures.

The sintering step itself is preferably carried out at a temperaturehigh enough to promote solid state diffusion of metal atoms from oneparticle to another to form the sintered bonds. For stainless steelmetal particulate, a temperature in the range of from about 1,600 toabout 2,550 degrees F., more preferably from about 1,900 to about 2,525degrees F., for a period of time of from about one to about eight hourshas been found adequate. Preferably, the sintering step is carried outunder a vacuum or in a pure hydrogen or other reducing atmosphere.

When lower melting materials are used, such as bronze, lower sinteringtemperatures may be used. For example, with bronze, temperatures in therange of from about 1,300 to about 1,900 degrees F. are adequate.

While the sintering step is preferably carried out at a temperature highenough to promote solid state diffusion as noted above, it can also becarried out using liquid phase sintering at relatively lowertemperatures, e.g., using silver with stainless steel particulate or tinwith copper.

In certain instances, it may be preferable to provide the driedstructure with solid hardware members, typically at each end of thestructure, and sinter them in situ, fusing the individual particles ofthe metal particulate to each other and fusing the solid hardwaremembers to adjacent particles of the metal particulate to provide thestructure with solid, closed pore or even porous end fitments, e.g., endcaps. For example, a completed filter element can be provided byinserting (positioning) end caps and/or other connecting fittings in theends of the container or mold prior to initiating rotation.Alternatively, the fitment can be inserted after the structure has beendried but prior to sintering. For example, it has been found that thedried structure has sufficient green strength (prior to sintering) thata threaded fitting can be screwed into it. During the subsequentsintering step, the metal particulate will form bonds between theindividual particles and the solid metal members, thereby forming acompleted filter element without the need for subsequent fabricationoperations. Internal support members, such as a spiral reinforcingspring, can also be positioned in the container or mold forincorporation into the formed structure.

Upon completion of the sintering step, the resulting structure is cooledand then removed from the furnace. Upon cooling, the seamless porousmetal structure will be easily removed from the container or mold due tothe formation of sinter bonds between the individual metal particles ofthe metal particulate.

An advantage of using a ceramic tube is that there is no need to employa releasing agent, e.g., a carbon mold releasing agent, to prevent theseamless structure from binding to the mold. The use of such releasingagents may contaminate the sintered structure and are difficult toremove. Accordingly, it is highly preferred to prepare the articles inaccordance with this invention without the use of a release agent or anyother coating material.

The cylindrical structure formed by the process above may be rolled,coined, swaged, welded, brazed, and/or resintered if desired. In thecase where a solid member must be attached by welding to the structure,it has been observed that welding the porous structure is improved dueto the uniform and non-stressed nature of the porous structure.

The porous metal articles in accordance with this invention typicallyhave nominal diameters ranging from about 1/2 to about 6 inches,preferably from about 1 to about 4 inches, and wall thicknesses rangingfrom about 0.005 to about 1 inch, more preferably from about 0.005 toabout 0.25 inch. As prepared, the lengths of the cylindrical structureswill typically range from about 1, or even less, to about 48, or evenhigher, inches. The prepared structures may be cut into any desiredlengths. Typically, the length to diameter (L/D) ratio of the structuresas prepared will be less than about 100, more typically in the range offrom about 1 to about 100. The porous metal articles in accordance withthis invention typically have F2 ratings at beta=100 (as hereinafterdefined) of from about 1 to about 50, preferably from about 3 to about40, micrometers. The sintered structures in accordance with thisinvention have relatively high voids volume at a given efficiencyrelative to other sintered structures of this general type due to theuniformity of the formed structures in accordance with this inventionand the relative absence of densty variations typically present insintered metal structures heretofore known to the art.

The F2 test used in making pore size measurements is a modified versionof the F2 test developed in the 1970s at Oklahoma State University(OSU). In the OSU test, a suspension of an artificial contaminant in anappropriate test fluid is passed through the test filter whilecontinuously sampling the fluid upstream and downstream of the filterunder test. The samples are analyzed by automatic particle counters fortheir contents of five or more preselected particle diameters and theratio of the upstream to downstream count is automatically recorded.This ratio is known in the industry as the beta ratio (β).

The beta ratio for each of the five or more diameters tested may beplotted as the ordinate against particle diameter as the abscissa,usually on a graph in which the ordinate is a logarithmic scale and theabscissa is a log² scale. A smooth curve is then drawn between thepoints. The beta ratio for any diameter within the range tested can thenbe read from this curve. Efficiency at a particular particle diameter iscalculated from the beta ratio by the formula:

    Efficiency, percent=100 (1-1/beta)

As an example, if beta=100, efficiency=99 percent.

In the modified F2 test, efficiencies in the range of from 1 to 20micrometers were determined using as a test contaminant a suspension ofAC fine test dust, a natural silicious dust supplied by the AC SparkPlug Company. Prior to use, a suspension of the dust in water was mixeduntil the dispersion was stable. Test flow rate was ten liters perminute per square foot of filter area.

The Bubble Point tests referred to in the examples below were carriedout at ambient temperature by submerging the appropriately end-cappedporous, cylindrical metal article to be tested in a liquid bath ofFilmex B (190 proof denatured ethyl alcohol available from AshlandChemical Company) to wet out all the pores. (Prior to being placed inthe bath, one end of the cylindrical structure was sealed while theother end was sealed to prevent liquid from entering the interior of thestructure and attached to a source of dry air.) Pressure was thenapplied to the interior of the structure (the cylinders tested were twoinches in diameter and four inches in length) and the pressure requiredfor the first or initial bubble of air to appear on the exterior surfaceof the cylinder was recorded. The pressure was then increased until aflow rate of 90,000 cubic centimeters of air per minute per square footof external surface area was flowing through the structure. For theexamples set out below, the distribution of bubbles on the exteriorsurface of the cylinders tested at this point was observed to be quiteuniform. The ratio of the pressure required to maintain the specifiedflow rate, i.e., 90,000, to the pressure required to form the initialbubble is a measure of the uniformity of pore size in the formedstructure. That is, the closer the ratio is to 1.0, the more uniform thepore size and the tighter the pore size distribution. To eliminate theeffect of the pressure drop of the structure itself on this ratio, theclean pressure drop (that is, in air with no wetting of the pores) atthe specified air flow rate of 90,000 was measured and subtracted fromthe pressure drop measured at the corresponding flow rate when thestructure was submerged in Filmex B prior to calculation of each of theratios.

GENERAL PROCEDURE FOLLOWED IN THE EXAMPLES

The porous articles in the examples below were prepared using thegeneral method described below. Where alternative procedures werefollowed in any of the following examples, such divergence from thegeneral procedure set out below is described with regard to theparticular example.

CARBOPOL 941 was combined with deionized water, the combination wasmixed until uniform dispersion of the CARBOPOL was obtained andaustenitic stainless steel particulate material (316L, having particlessizes as specified below) was then added and mixed to provide a uniformdispersion of the metal particulate in the liquid medium having thedesired composition. The amounts of the CARBOPOL 941, water and metalparticulate are specificed in Table 1 below.

An open, cylindrical ceramic container or mold having the compositionset out under (e) above, having an internal diameter and a length asspecified in the examples was sealed at one end and then completelyfilled with the particular stabilized suspension. The open end of theceramic container or mold was then sealed, enclosing the suspensionwithin. The container was then mounted horizontally on a machine lathe,the lathe was started and the rate of rotation brought up to the rate ofrevolution indicated in each example. Following a period of from about 3to about 5 minutes, the power to the lathe was terminated and the lathewas allowed to rotate to a stop. The container was then removed from thelathe, opened and supernatant fluid poured off. Alternatively, thesupernatant fluid can be removed while the container is rotating byproviding drain caps in the end caps which can be opened while thecontainer is rotating (after the metal particulate has been laid down onthe interior surface of the mold). The container with the formedstructure therein was then placed horizonally in an oven at 300 to 350degrees Fahrenheit from about 30 to about 45 minutes until the powderwas dry.¹ The container was then placed vertically within a vacuumfurnace and the dried metal structure therein was subjected to asintering temperature (as specified below) for a period of 4 hours. Thecontainer was then cooled and removed from the furnace. The porous metalstructure was then removed from the ceramic tube for testing. Theresults are set out in Table 2 below:

                  TABLE 1                                                         ______________________________________                                                   Nominal Average                                                                            Ratio of Metal                                        Example    Particle Size                                                                              Powder/Carrier.sup.1                                  ______________________________________                                        1          20μ       4.1:1                                                 2           8μ       7.3:1                                                 ______________________________________                                         .sup.1 This ratio refers to the ratio of the weight of metal particulate      to the weight of the carrier, i.e., the CARBOPOL 941 and deionized water.     In all of these examples, the CARBOPOL 941 was present in an amount of        0.35 weight percent (based on the deionized water).                      

The pressure required to obtain the first or initial Bubble Point inFilmex B as well as the pressure required to maintain a flow rate of90,000 as described above, were determined and the ratio of the latterpressure to the pressure measured for the initial Bubble Point (aftercorrection for the clean pressure drop) was calculated. The results areset out in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                   Ratio of Pressure to Pressure                                      Example    of First Bubble Point                                              ______________________________________                                        1          2.1:1                                                              2          2.0:1                                                              ______________________________________                                    

The method of preparation of the porous article denotes above as Example1 will now be described.

Using the General Procedure described above, a seamless porous article48 inches long and having an outside diameter of 0.460 inch was preparedfrom austenitic stainless steel particulate material (316L) having amean particle diameter of about 20 micrometers using a ceramic mold 48inches long and with an inside diameter of 0.46 inch. The final rate ofrotation (3,200 rpm) was sufficiently high to provide a G force of 67.After finishing by sintering at 2,050 degrees F. using the GeneralProcedure described above, the structure was determined to have an F2rating at beta=5,000 of 5 micrometers (an estimated F2 rating atbeta=100 of 3 micrometers).

In like manner, another seamless porous article denoted above as Example2 was prepared from austenitic stainless steel particulate matter(316L), but having a mean particle diameter of about 8 micrometers usingthe general procedure described above. In this example the ceramic moldhad an inside diameter of 0.875 inch and was 18 inches long. Theresulting product was 18 inches long, with an outside diameter of 0.875inch and a wall thickness of 0.25 inch. The final rate of rotation wassufficient to generate a G force of 75 at the interior wall of thestructure and the sintering was also carried out at 2,050 degrees F.

The ratios of the pressures at the specified flow rate, i.e., 90,000, tothe pressure required to form the first or initial Bubble Point werebelow 2.5. These relatively low ratios reflect the substantially uniformpore structures of the porous articles prepared by the method inaccordance with the invention. As used herein for purposes of describingthe uniform pore characteristics of the porous articles in accordancewith the invention, the term "Bubble Point ratio" refers to the ratio ofthe pressure required to maintain a flow rate of 90,000 cubiccentimeters per square foot (in the test method set out above and withthe clean pressure drop at a flow rate of 90,000 cubic centimeters ofdry air per minute per square foot of surface area subtracted from themeasured value prior to calculating the ratio) to the pressure requiredfor the first bubble of air to appear (as described in the test methodset out above).

The results described above demonstrate that the porous articles inaccordance with this invention provide structures with uniform porecharacteristics. Further, the articles, because of their method ofmanufacture, do not suffer from the drawbacks associated with weldedstructures.

I claim:
 1. A method of manufacturing a seamless, hollow, porous articlecomprising:(a) rotating a mold containing a stabilized suspension of ametal particulate at a rate and for a time such that said particulate isseparated from said suspension and distributed on the interior wall ofsaid mold, thereby forming a structure conforming to the interior wallof said mold, the rate of rotation being sufficiently high that at leastabout 65 Gs up to 100 Gs of acceleration is achieved at the interiorwall of said structure; (b) drying said structure to provide a driedstructure having green or unsintered strength; (c) sintering the driedstructure to remove volatile material and fuse the individual particlesof said particulate to each other to form a seamless, hollow, porousarticle; and (d) removing the seamless, hollow, porous article from saidmold.
 2. The method of claim 1 wherein said mold is substantiallycompletely filled with said stabilized suspension.
 3. The method ofclaim 2 wherein the rate of rotation is at such that at least about 70Gs of acceleration is achieved at the interior wall of said article. 4.The method of claim 3 wherein the weight ratio of said metal particulatein said stabilized suspension to the other components therein is fromabout 8:1 to about 1:1.
 5. The method of claim 4 wherein said metalparticulate is stainless steel, said mold is ceramic and said sinteringis carried out at a temperature in the range of from about 1,600 toabout 2,550 degrees F.
 6. The method of claim 5 wherein said metalparticulate is austenitic stainless steel, the weight ratio of saidmetal particulate in said stabilized suspension to the other componentstherein is from about 4.5:1 to about 3.5:1 and said sintering is carriedout at a temperature in the range of from about 1,900 to about 2,525degrees F.
 7. The method of claim 6 wherein said stabilized suspensioncomprises (1) water, (2) a stabilizing/binding agent comprising apolyacrylic acid and (3) austenitic stainless steel particles havingaverage particle sizes in the range of from about 25 to about 150micrometers, the weight ratio of said metal particulate in saidstabilized suspension to the other components therein is in the range offrom about 3.5:1 to about 4.5:1, said mold has an inside diameter in therange of from about 1 to about 4 inches and an L/D of from about 1 toabout 100, said stainless steel particles have a coefficient of thermalexpansion at least one and one-half times as great as said mold, therotation of said mold is carried out with said mold in a horizontalposition, said drying is carried out with said mold in a horizontalposition, said sintering is carried out with said mold in a verticalposition and the formed seamless, hollow porous article has a wallthickness of from about 0.005 to about 0.25 of an inch, an F2 rating atbeta equal 100 of from about 1 to about 50 micrometers and a BubblePoint ratio of greater than 1.5 up to 2.5.
 8. The method of claim 1wherein the rotation of said mold is carried out with said mold in ahorizontal position, said drying step is carried out with said mold in ahorizontal position and said sintering is carried out in a verticalposition.
 9. The method of claim 1 wherein steps (a) and (b) arerepeated at least once with at least a second stabilized suspension of asecond metal particulate having a different particle size distributionthan that of the first stabilized suspension prior to carrying out step(c) thereby forming a seamless, hollow, porous article with a layeredstructure.
 10. The method of claim 1 wherein one or more fitments isinserted into said mold prior to said sintering.
 11. A method formanufacturing a seamless, hollow, porous metal article having a gradedpore structure comprising:(a) rotating a mold containing a stabilizedsuspension of a metal particulate having a distribution of particlesizes dispersed in a fluid medium, said mold having a coefficient ofthermal expansion less than that of said metal particulate, at a firstlower rate to first separate larger size particles from said suspensionand distribute them over the interior wall of said mold; (b) rotatingsaid mold at at least one higher rate of revolution to separate smallersize particles from said suspension and distribute them over thepreviously distributed larger particles, thereby forming a structureconforming to the interior wall of said mold, said mold being rotated ata rate such that at least about 65 Gs up to about 100 Gs of accelerationis achieved at the interior wall of said structure; (c) drying saidstructure to provide a dried structure having green or unsinteredstrength; (d) sintering the dried structure to remove volatile materialand fuse the individual particles of said particulate to each other toform a seamless, hollow, porous article having a graded pore structure;and (e) removing the seamless, hollow, porous article having a gradedpore structure from said mold.
 12. The method of claim 11 wherein saidmold is substantially completely filled with said stabilized suspension.13. The method of claim 12 wherein the rate of rotation is at such thatfrom about 70 to about 80 Gs of acceleration is achieved at the interiorwall of said structure.
 14. The method of claim 15 wherein the weightratio of said metal particulate in said stabilized suspension to theother components therein is from about 8:1 to about 1:1.
 15. The methodof claim 14 wherein said metal particulate is stainless steel, said moldis ceramic and said sintering is carried out at a temperature in therange of from about 1,600 to about 2,550 degrees F.
 16. The method ofclaim 15 wherein said metal particulate is austenitic stainless steel,the weight ratio of said metal particulate in said stabilized suspensionto the other components therein is from about 4.5:1 to about 3.5:1 andsaid sintering is carried out at a temperature in the range of fromabout 1,900 to about 2,525 degrees F.
 17. The method of claim 16 whereinsaid stabilized suspension comprises (1) water, (2) astabilizing/binding agent comprising a polyacrylic acid and (3)austenetic stainless steel particles having average particle sizes inthe range of from about 8 to about 60 micrometers, the weight ratio ofsaid metal particulate in said stabilized suspension to the othercomponents therein is in the range of from about 3.5:1 to about 4.5:1,said mold has an inside diameter in the range of from about 1 to about 4inches and an L/D of from about 1 to about 100, said stainless steelparticles have a coefficient of thermal expansion at least one andone-half times as great as said mold, the rotation of said mold iscarried out with said mold in a horizontal position, said drying iscarried out with said mold in a horizontal position, said sintering iscarried out with said mold in a vertical position and the formedseamless, hollow porous article has a wall thickness of from about 0.005to about 0.25 of an inch, an F2 rating at beta equal 100 of from about 3to about 40 micrometers and a Bubble Point ratio of greater than 1.5 upto about 2.5.
 18. The method of claim 11 wherein the rotation of saidmold is carried out with said mold in a horizontal position, said dryingstep is carried out with said mold in a horizontal position and saidsintering is carried out in a vertical position.
 19. The method of claim11 wherein one or more fitments is inserted into said mold prior to saidsintering.
 20. A seamless, hollow porous metal article of substantiallyuniform diameter, wall thickness and pore structure comprising metalparticulate in which the individual particles of said particulate arebonded to each other and said article has a Bubble Point ratio ofgreater than 1.5 up to about 2.5.
 21. The porous metal article of claim20 wherein said article has a graded pore structure.
 22. The porousmetal article of claim 21 wherein said particulate is stainless steel.23. The porous metal article of claim 22 wherein said article has a wallthickness of from about 0.005 to about 1 inch.
 24. The porous metalarticle of claim 23 wherein said article has an F2 rating at beta equals100 of from about 1 to about 50 micrometers.
 25. The porous metalarticle of claim 24 wherein said stainless steel is austenitic stainlesssteel and said article has a wall thickness of from about 0.005 to about0.25 of an inch.
 26. The porous metal article of claim 25 wherein saidarticle comprises a finer pored outer layer and a coarser pored innerlayer.
 27. The porous metal article of claim 26 wherein said outer layeris about 0.015 inches thick and said inner layer is about 0.040 inchesthick.
 28. The porous metal article of claim 27 wherein said article hasa nominal 4 inch outer diameter, said outer layer comprises metalparticulate having a nominal particle size of -325, and said inner layercomprises metal particulate having a nominal particle size of -200,+325.
 29. The porous metal article of claim 27 wherein said article hasone or more fittings sinter-bonded to said article.
 30. The porous metalarticle of claim 24 wherein said article has an F2 rating at beta equals100 of from about 3 to about 40 micrometers.